This invention relates to a ferro-electric liquid crystal electro-optical device, for example, a display device and a shutter for a printer.
A liquid crystal panel has many exceeding advantages such as small and thin size, and its electric consumption is low. Therefore, the liquid crystal panel is largely used in indicators of watches and calculators.
The liquid crystal used in these displays is thermotropic liquid crystal, and therefore, assumed many kinds of liquid crystal phases in a given temperature range. These liquid crystal phases are classified into two categories, the nematic phase (hereinafter referred to as N) which does not have the layer structure, and the smectic phase (hereinafter referred to as Sm) which has the layer structure.
Sm is classified again into one-axis smectic A phase (hereinafter referred to as Sm A), and two-axis smectic C phase (hereinafter referred to as Sm C). The thickness of the layers approximately corresponds to the length of one molecule of a liquid crystal.
FIGS. 2a, 2b and 2c show typical diagrams of the molecule orientation of N, Sm A, and Sm C. FIG. 2a shows N, FIG. 2b shows Sm A, and FIG. 2c shows Sm C.
Furthermore, if the liquid crystal molecules have the asymmetric carbon and do not have the racemic body, the liquid crystal assumes the spiral structure. In the case of N, the long axis of liquid crystal molecule is along the thin layers, and is also oriented to the same direction. Then N becomes the chiral nematic in which each layer holds same direction of the molecules, and adjacent layers are with each other.
FIG. 3 shows a typical molecule orientation of the chiral nematic. In this case, the molecules 8 are spirally oriented and the normal direction of the layer is identical to the spiral axis.
FIG. 4a shows a typical molecule orientation of the chiral smectic C phase (hereinafter referred to as Sm C*).
Explanation of Sm C* as is follows;
The long axis of liquid crystal molecule 8 (hereinafter referred to as molecule axis) in one layer inclines .theta. degrees relative to the normal direction of the layer, and this angle is identical in any layer.
FIG. 4b shows the relation between the molecule axis and the normal direction of the layer.
Meanwhile, when viewing the molecule orientation of Sm C* from the normal direction of the layers, the direction angle .phi. rotates in successive layers at a fixed value (FIG. 4a shows the change where the direction angle .psi. rotates 45.degree. at each step), and the molecule orientation forms a spiral structure.
In general, Sm C* does not only form a spiral construction, but also has an electric dipole 9 in an orthogonal direction to the molecule axis, and shows ferroelectric characteristic.
The ferroelectric liquid crystal was synthesized and proved in 1975, by Meyer et al (J.de. Phys. 36, 69, 1975).
The liquid crystal synthesized at that time is so called DOBAMBC, (2-methyl butyl P-[(P-n-decyloxybenzylidene)amino] ##STR1## is now largely used in the study of the ferroelectric liquid crystal.
Sm C* has the spiral structure as stated above, and the pitch of the spiral differs by the type of liquid crystal, but is usually a several .mu.m.
When the liquid crystal which assumes Sm C* is poured into a cell which has a gap of about 1 .mu.m, thinner than the pitch of the spiral, the spiral structure disappears.
The molecule orientation structure after the disappearance of the spiral structure, is shown in FIG. 5, with the geometric relation with the cell plates 10.
The liquid crystal molecules 8 are parallel to the cell plates 10. Therefore, the liquid crystal molecules 8 are oriented so that the molecule axis is parallel to the plates, and inclines .theta. degrees from the normal direction of the layer.
At this point, the normal direction of the layer is parallel to the plates, thus, the layer is formed orthogonally relative to the plates. When the molecule axis inclines .theta. degrees from the normal direction of the layer, the domain which is inclined .theta. degrees in a clock-wise direction from the normal, and the domain which is inclined .theta. degrees in anticlock-wise direction, exist together.
Sm C* liquid crystal molecule generally has an electric dipole 9 which is orthogonal to the molecule axis. If an electric dipole is oriented upwardly relative to the cell plates in one domain, another dipole is oriented downwardly in the other domain.
When an electric field is applied between these cell plates, the liquid crystal molecule of the whole cell is oriented in a bi-stable position +.theta. or -.theta. inclined from the normal direction of the layer as shown in FIGS. 6a and 6b (+, - are determined according to the direction of the electric dipole). Hereinafter, we refer to these positions as +.theta. position, and -.theta. position.
When the electric field is applied oppositely to the liquid crystal layer, the liquid crystal molecule moves either from +.theta. position to -.theta. position, or from -.theta. position to +.theta. position according to the direction of the applied electric field. As the whole molecules of the cell orient at either +.theta. position or -.theta. position, this phase structure is Sm C. The Sm C phase is caused by the dissapearance of the spiral structure and by making the cell gap thinner.
However, because of an inherency of the original spiral structure, when this Sm C moves from the .+-..theta. position to the opposite position, the molecule moves along the cone as shown in FIG. 4b. Usually, Sm C does not undergo such movement as this, even when the electric field is applied thereto.
By suitably selecting the polarity of the electric field, by moving the liquid crystal molecules between +.theta. position and -.theta. position, and by attaching polarizers on the two cell plates, this cell can be used as a display element.
FIGS. 6a and 6b show the relation between the two polarizers and the .+-..theta. positions of liquid crystal molecule, when using the cell as a display element.
Referring to FIG. 6a , the polarizing axis 12 of the polarizer on the incidence side is matched to +.theta. position. The polarizer on the outgoing or transmitting side has the polarizing axis 11 displaced by 90.degree. from the axis 12 of the polarizer on the incidence side.
As shown in FIG. 6a, when the liquid crystal molecule is in the +.theta. position, the light polarized by the polarizer on the incidence side reaches the polarizer on the outgoing side without changing the polarizing direction, but as the polarizers cross at right angle, the light is not radiated from the outgoing side.
This condition is the dark condition. On the other hand, if the liquid crystal molecule moves to the --position, the light is radiated from the outgoing side according to the birefringency of the liquid crystal.
If .theta. is 22.5.degree. and the thickness of cell is appropriate, most of the light align with the polarizing direction of the outgoing side polarizer, and the cell becomes the bright condition.
To obtain an ideal display condition as described above, the relation between the cell thickness d and the anisotropy .DELTA.n of the refractive rate n of the liquid crystal, shown as below, is necessary: EQU d=(2n-1).alpha./.DELTA.n
Where,
.alpha.=C.pi./.omega., PA1 C=light speed PA1 .omega.=angular frequency of light
FIG. 6b shows the case when the directions 11, 12 of the polarizers of both incidence and outgoing sides, are the same.
In this case, +.theta. position corresponds to the bright condition, and the -.theta. position corresponds to the dark condition. The relation between the cell thickness d and .DELTA.n is the same as the above formula. An ideal bright-dark switching is attained when .theta.=22.5.degree..
This type of the display element was first announced by Clark and Lagerwall (Appl. Phys. lett. 36, 899, 1980).
They stated that a display element with a thinner cell and two attached polarizers, has such characteristics as follows;
(1) High-speed response of .mu. sec. order
(2) Memorizing characteristic
(3) Desirable threshold value characteristic
Among these characteristics, the memorizing characteristic still exists in the inventive cell, and this characteristic enables the cell to maintain the bright or dark condition even when the electric field is turned off, after the high-speed response and after the electric field is applied to the liquid crystal to switch between +.theta. or -.theta. position.
According to our experiments, however, the existence of the desirable threshold value was not confirmed.